Not only the fat content but also the composition of fatty acids (FAs) in stored triglycerides might be of interest in the research on nonalcoholic fatty liver disease and nonalcoholic steatohepatitis. In this study, a novel reconstruction approach is proposed that uses theoretical knowledge of the chemical structure of FAs to simultaneously quantify the fat fraction (FF) and the FAs composition (chain length cl, number of double bonds ndb, and number of methylene-interrupted double bonds nmidb) from multiple gradient echo images. Twenty phantoms with various fat contents (FF = 9–100%) and FA compositions (cl = 12.1–17.9, ndb = 0.23–5.10, and nmidb = 0.04–2.39) were constructed and imaged in a 3-T Siemens scanner. In addition, spectra were acquired in each phantom. Slopes and “standard deviations from true values” were used to investigate the accuracy of the two methods. The imaging method holds well in a comparison to the previously suggested spectroscopy method and showed similar overall accuracy. The in vivo feasibility was demonstrated in the thigh adipose tissue of a healthy volunteer. In conclusion, our developed method is a promising tool for FF and FA composition quantification. Magn Reson Med, 2013. © 2012 Wiley Periodicals, Inc.